1. Field of the Invention
This invention pertains to the field of hydrocarbon processing. The invention pertains, in particular, to a fluid catalyst cracking process in which the method of contacting of the oil feed with a recycled spent catalyst and freshly regenerated catalyst results in an improved product distribution from the cracking operation.
2. Description of the Prior Art
In the fluid catalyst cracking process, liquid or partially vaporized hydrocarbon feed stock generally contacts hot freshly regenerated catalyst in the lower section of a riser reaction zone. The amount of regenerated catalyst employed is sufficient to supply the heat of vaporization of the oil feed, the endothermic heat of cracking, and the sensible heat required to carry out the cracking reaction at the desired operating temperature. The mixture of oil vapor and catalyst flows up through the high velocity riser, where cracking to the desired lighter products and to coke deposited on the catalyst occurs. The desired conversion of the oil feed may occur completely in the riser, or this conversion may be incomplete and the additional desired conversion may occur in a low velocity fluid catalyst bed superimposed on top of the riser reaction section. The vapor from the cracking section may flow directly to a cyclone separator, but more generally into a disengaging space and then to a cyclone separator system where hydrocarbon vapors are separated from the spent catalyst. The separated hydrocarbon vapors then flow to a main fractionator where they are separated into such typical fractions as a light gas and gasoline overhead, light cycle oil and heavy cycle oil sidestreams, and a bottom stream which contains fine catalyst which was not collected in the cyclone separator system. The bottom stream is settled in the bottom section of the fractionator or in a separate settler to produce a decanted oil fraction and a slurry oil which is returned to either the inlet or outlet of the reaction zone to recover the entrained catalyst fines. Some of the products from the main fractionator may flow to additional recovery equipment where additional separation and purification steps are carried out. The spent catalyst separated from the hydrocarbon vapors in the disengaging zone and cyclone separator system flows to a stripper where the countercurrent flow of steam removes absorbed and interstitial hydrocarbons from the catalyst. The stripped catalyst flows through a standpipe and a controlled slide valve either directly as a dense phase to the regenerator or it may be transported by air to the regenerator. The catalyst in the regenerator is usually maintained as a dense fluid bed, although transport type regeneration or multiple bed type regeneration may be employed. In the regenerator, the catalyst contacts oxygen containing gas which burns the freshly deposited coke from the catalyst.
Flue gas from the burning of coke flows through a dilute phase disengaging zone to a cyclone separator system where the entrained catalyst is recovered and returned to the regenerator bed. The flue gas usually contains carbon monoxide, carbon dioxide, steam, nitrogen and a small amount of oxygen. This gas flows to a flue gas boiler where the carbon monoxide is burned and the resultant heat recovered. Alternatively, the conditions in the catalyst regeneration zone may be so controlled that essentially complete burning of the carbon monoxide is obtained. This can be accomplished by extensive burning of the carbon monoxide in the dilute phase section of the regenerator or by the use of special catalysts containing an additive which promotes the burning of the carbon monoxide to carbon dioxide within the catalyst bed. The amount of air fed to the regenerator is sufficient to burn all the coke deposited in the reaction section and to maintain the residual carbon on the regenerated catalyst at a low level. The freshly regenerated catalyst flows from the regenerator bed through a standpipe and controlled slide valve to the bottom section of the reaction zone where, as previously mentioned, it contacts the feed stock to be cracked.
In these typical fluid catalytic cracking operations, the virgin feed stock to be cracked always contacts the freshly regenerated catalyst with the result that the highly reactive coke producing contaminants in the feed readily deposit on the catalyst and destroy a substantial portion of the activity of the catalyst before any extensive cracking of the feed occurs. As a result, the bulk of the desired conversion of the oil feed is carried out with a partially deactivated catalyst with the result that more severe operating conditions are required and the selectivity of the cracking operation to produce the desired gasoline product is impaired.
In the practice of my invention, the feed is first contacted with a partially spent catalyst, and then with a freshly regenerated catalyst. As a result of this method of contacting, the highly reactive contaminants in the feed are deposited on the spent catalyst with the result that the bulk of the desired cracking is then subsequently carried out with a highly reactive and selective regenerated catalyst free of these oil contaminants. The so-called spent catalyst contacted with the fresh feed is still sufficiently active to react with the contaminants in the feed without resulting in any substantial cracking of the bulk of the hydrocarbon in the feed. Carrying out the major portion of the cracking with a catalyst essentially free of these undesirable contaminants in the feed results in a more extensive and more selective cracking operation in which higher yields of the more desired products are obtained.
The following listing of patents include relevant teachings considered by applicant as prior art:
U.S. Pat. No. 2,312,230 -- Class 208/157
U.S. Pat. No. 2,439,811 -- Class 208/74
U.S. Pat. No. 2,487,132 -- Class 208/150
U.S. Pat. No. 2,700,015 -- Class 208/150
U.S. Pat. No. 2,847,364 -- Class 208/59
U.S. Pat. No. 2,892,773 -- Class 208/213
U.S. Pat. No. 2,965,454 -- Class 23/288
U.S. Pat. No. 3,071,538 -- Class 208/120
U.S. Pat. No. 3,182,011 -- Class 208/78
U.S. Pat. No. 3,344,060 -- Class 208/140
U.S. Pat. No. 3,380,911 -- Class 208/74
U.S. Pat. No. 3,679,576 -- Class 208/74
U.S. Pat. No. 3,888,762 -- Class 208/120